1. Field of the Invention
The subject invention relates to an apparatus for measuring the refractive index of index matching gels, and other substances having a variable refractive index.
2. BRIEF DESCRIPTION OF THE PRIOR ART
In the field of fiber optic devices where fiber cables must be spliced, it is known to splice fiber ends by way of index matching gels having an index of refraction equal to the index of refraction of the fibers to be spliced. It is also known to insert index matching gels at ends of fiber optic connectors to minimize losses at the interface of the connectors. Such an interconnection then is dependent on the accuracy to which the gel is matched to the fiber. If the gel does not match the fibers to be spliced, the interface between the two fibers will have losses associated thereat, due to the reflection caused by the difference in refractive index between the fiber and gel. Such losses could be detrimental to the electronic system for which the fibers are used.
Devices are known to measure the refractive index of index matching gels, one such device being known as the Abbe refractometer. One of the drawbacks to the Abbe refractometer is that the Abbe device can only give digital read outs, and cannot give measurements for varieties of temperatures, due to the method of making the measurements, as disclosed above.
Another disadvantage is that the Abbe refractometer cannot detect the extent to which the gel is mixed. Index matching gels are comprised of an oil and glass filler particles suspended in the oil. The glass filler particles should be selected such that the refractive index of the particles at all temperatures is the same as the refractive index of the fiber. The glass filler particles should also be thoroughly mixed in the oil to make a homogeneous gel. If the particles are not thoroughly mixed and clustered together, there will be light scattering, leading to power losses. As mentioned above, the Abbe refractometer cannot detect the extent to which the gel is mixed, but rather can give only an instantaneous value of the index of refraction for a given sample. A different sample, taken from another location may well have a different index of refraction. The index of refraction may also be time variable as the glass particles, if not properly mixed, tend to spin or rotate in the oil, and the index of refraction can change as the collected glass particles rotate.
Another disadvantage is that the Abbe refractometer measures the refractive index in the visible range. As the refractive index of matching gels is wavelength dependent, it would be advantageous to measure the refractive index of the gels at the wavelengths which are to be used, i.e. 1300 and 1500 nm. The invention explained herein measures the refractive index at such wavelengths.